The phenomenon of the intumescence is known: when film of paint is subjected to the action of a flame, at first it changes into a plastic mass which then starts to swell because of the formation of incombustible gases which remain trapped in the film causing the latter to expand up to about 50 times the original thickness. The cellular layer that thus is formed solidifies into a spongy, practically incombustible mass which isolates the substrate to be protected and hinders the admission of air.
In order to obtain the phenomenon of intumescence, in general there are needed three components:
(1) a source of carbon, consisting of a carbohydrate such as starch or glucose, or of a polyvalent alcohol such as pentaerythritol or its dimer or trimer;
(2) a dehydrating agent capable of promoting the formation of a carbonaceous mass starting from carbohydrate and in general consisting of a source of phosphoric acid, such as ammonium or melamine phosphate: the dehydrating agent decomposes while releasing phosphoric acid which esterifies the hydroxyl groups of the carbohydrate under formation of a phosphoric ester which subsequently chars;
(3) an intumescing agent that decomposes while releasing incombustible gases that expand the film of paint, said agent consisting either of nitrogen containing compounds such as urea, melamine, dicyandiamide, or also of a partially halogenated organic compound such as a paraffin wax chlorinated at 70% by weight of chlorine.
The three above indicated components cannot be chosen arbitrarily; the dehydrating agent must have a decomposition temperature near the decomposition temperature of the intumescing agent. If, for instance, the intumescing agent decomposes at a temperature considerably greater than that at which the dehydrating agent decomposes, then the film of paint solidifies to a carbonaceous mass before it expands, and consequently this hinders the formation of a multicellular layer.